Crystalline aluminosilicate zeolites, commonly referred to as "molecular sieves", are now well known in the art. They are characterized by their highly crystalline structure and uniformly dimensioned pores and are distinguishable from each other on the basis of composition, crystal structure, topology, absorption properties, surface area, pore volume, and the like. Zeolite materials, both natural and synthetic, have been demonstrated to have catalytic properties for various types of hydrocarbon conversion and chemical processing. It is often advantageous to dealuminate these materials in order to improve their process performance. Performance measures include product selectivity, product quality, and catalyst stability. Dealumination is also practiced in order to permit insertion of transition metals such as titanium into the lattice framework of the zeolite; the inserted metal thus takes the place of the aluminum which has been removed. Near quantitative aluminum removal is frequently desired, both to enable a high concentration of transition metal to be inserted and to avoid the deleterious effects that residual aluminum may cause. Aluminum sites in a zeolite framework, for example, often have a strongly acidic character and thus may catalyze undesired side reactions.
Conventional techniques for zeolite dealumination include hydrothermal treatment, mineral acid extraction, and chemical treatment with silicon tetrachloride or a chelating agent such as EDTA or a dicarboxylic acid. Such methods are reviewed in J. Scherzer, "The Preparation and Characterization of Aluminum-Deficient Zeolites", Catalytic Materials, Chapter 10, pp. 157-200 (1984). A listing of some of the patent art in this field may be found in U.S. Pat. No. 5,019,543. However, it is well known that such treatments are limited, in many cases, in the extent of dealumination by the onset of crystal degradation and loss of sorption capacity. One group of workers in the field has recently observed that ". . . the experimental conditions appear critical and must be systematically adjusted to the composition, structure, and/or texture of the parent material in order to preserve the integrity of the crystallinity" and that "complete removal of the framework aluminum requires repeated treatments" [Lami, et al., Microporous Materials, 1, 237-245(1993)]. According to U.S. Pat. No. 3,691,099, "the exposure of . . . aluminosilicates . . . to strongly acidic environment may produce undesirable changes in both physical structure and chemical composition by promoting loss of surface area, collapse of crystal structure or deactivation of acidic cation exchange sites."
Recently, in U.S. Pat. No. 5,310,534, a process for the dealuminization of large pore zeolites has been proposed involving leaching of the raw zeolite containing the structuring agent ("organic structurant") used to form the zeolite. The leaching, according to the patent, is accomplished through the use of "a strong organic acid" (formic acid, trichloroacetic acid, and trifluoroacetic acid being the only named organic acids) or "a strong inorganic acid". The patent stresses the need to have the structuring agent still present in the zeolite during acid leaching in order to preserve a high degree of crystallinity. The applicants submitted additional evidence during prosecution of the patent demonstrating that when nitric acid is used to treat a zeolite from which the structuring agent had first been removed, significant crystallinity losses result. The results of the examples described in European Pat. Publication No. 95,304 similarly suggest that treatment of such a zeolite with hydrochloric acid in an attempt to form beta zeolites having a high silica/alumina ratio also leads to reduced crystallinity.
It is apparent, therefore, that a process which would accomplish a high degree of dealumination using either protonated (calcined) zeolite or raw zeolite and a minimum number of treatment steps while preserving the crystallinity and ordered structure of the zeolite would be highly desirable.